1. Field of the Invention
The present invention relates to an image forming apparatus which corrects a misregistration between a plurality of toner images transferred onto a recording medium.
2. Description of the Related Art
Conventionally, there is known an electrophotographic color image forming apparatus in which toner images formed on a plurality of photosensitive members are transferred onto a recording medium so that a color image is formed on the recording medium. Because it is very difficult to eliminate manufacturing variations of components and an assembly error of the color image forming apparatus, there may occur a color misregistration (out of color registration) between images formed on the recording medium. Therefore, the image forming apparatus is equipped with a structure configured to correct the color misregistration.
As one method of correcting the color misregistration, the following color misregistration correction control is known. In order to detect the color misregistration, a toner pattern for position detection purpose (toner patch) is formed on each of the plurality of photosensitive members. The toner pattern for position detection purpose formed on each of the plurality of photosensitive members is transferred onto an image bearing member such as an intermediate transfer member or a recording medium transport member. A position of the toner pattern for position detection purpose which is transferred onto the image bearing member is detected, and an amount of color misregistration is obtained on the basis of the detection result. On the basis of the obtained amount of color misregistration, the color misregistration correction control is performed so as to correct each position at which each color toner image is formed. An optical sensor is used for detecting the position of the toner pattern for position detection purpose (see Japanese Patent Application Laid-Open No. 2008-96744). The optical sensor detects the position of the toner pattern for position detection purpose by irradiating the image bearing member and the toner pattern for position detection purpose with light and by detecting reflection light.
If the toner patterns for position detection purpose are formed to be overlapped at a portion having locally low reflectance due to a flaw or dirt which occurs in a manufacturing process of the image bearing member such as the intermediate transfer member or the recording medium transport member or occurs due to deterioration over time, a correct formation position of the toner pattern for position detection purpose may be detected incorrectly. The image forming apparatus described in Japanese Patent Application Laid-Open No. 2008-96744 can prevent an influence of the flaw or dirt on the color misregistration correction by performing a masking process of correcting intensity of the reflection light from the flaw or dirt so that the flaw or dirt is not recognized as the toner pattern for position detection purpose.
Further, if the reflectance of a surface of the image bearing member is decreased by deterioration of the image bearing member such as the intermediate transfer member or the recording medium transport member, the light intensity of a light emitting portion is controlled to be increased for reliably detecting the toner pattern for position detection purpose (see Japanese Patent Application Laid-Open No. 2007-78874).
However, the masking process described in Japanese Patent Application Laid-Open No. 2008-96744 cannot be performed on a detection result in a vicinity of a position where the toner pattern for position detection purpose is formed. Therefore, the following problem occurs. Specifically, if the toner patterns for position detection purpose are formed to be overlapped on a part of the flaw or dirt on the image bearing member in the transport direction of the image bearing member, a width of the toner pattern for position detection purpose in the transport direction of the image bearing member is detected to be larger than the real width. If the width of the toner pattern for position detection purpose is detected to be larger than the real width, detection accuracy of the position at which the toner pattern for position detection purpose is formed is decreased.